(1) Field of the Invention
The present invention relates to an optical pickup apparatus, such as an optical disc or an optical card, which records/reproduces or erases information in an optical medium or a magneto-optical medium.
(2) Description of the Related Art
Optical discs are used, as a high-density and large-volume recording medium, in a variety of fields, for storing and distributing not only music but also video and various kinds of data, and the market for optical discs continues to expand.
Within such an optical disc apparatus, a section which serves to retrieve information from an optical disc and record into an optical disc is an optical pickup apparatus. This optical pickup apparatus is generally configured of: a semiconductor laser device, which is a light source; a condenser optical system which condenses a light beam from the semiconductor laser device onto the optical disc; a diverging optical system which diverges a part of the laser beam that is reflected on the optical disc, passes through the condenser optical system, and returns; and a light receiving element, which photo-detects the diverged light beam.
DVD devices, which have spread excessively in recent years, have specifications for reproducing/recording CDs and DVDs. Therefore, a DVD device includes a red semiconductor laser device and an infrared semiconductor laser device as semiconductor laser devices.
Recently, in semiconductor laser devices, use of a dual wavelength laser element, in which two laser elements are integrated into a single package (monolithically or in hybrid; see Patent Reference 1 for an example of monolithic integration), continues to be the future technological trend of optical pickup apparatuses. (Patent Reference 1: Japanese Patent Application No. 11-186651.)
In addition, this technological trend is the same for next-generation optical disc apparatuses that record/reproduce using a blue light source, which are presently continuing to be developed as high-density discs. In other words, an optical pickup apparatus found in a next-generation optical disc apparatus must carry out reproduction/recording of conventional DVDs and CDs. Therefore, red and infrared light sources are needed in addition to a blue light source, and in the future it will be necessary to implement a light source that has integrated the aforementioned light sources into a single device and an optical pickup apparatus that uses that light source.
Because the semiconductor laser device that integrates light sources of a plurality of wavelengths exists with those light sources at a certain distance, an optical axis of each light source differs. However, the optical pickup must detect a servo signal for a tracking servo of that objective lens, or an address signal for detecting an address signal, in order to read/write information in the optical disc.
A generally-used method for obtaining these signals involves: dividing a light beam, which is condensed by the objective lens onto the optical disc, reflected, and captured back into the objective lens, bilaterally on a track line of the optical disc; detecting each of the divisions; and finding the differential.
However, as indicated by FIG. 1, this plurality of light sources is arranged in the bilaterally-divided direction, or, in other words, in the direction perpendicular to the track line of the optical disc. Therefore, the optical axis of the light beam outgoing from the light sources and the optical axis of the reflected light differs with each light source within the optical system, and it is thus difficult to obtain an ideal bilaterally-divided signal with a simple bilaterally-divided optical element. It is possible to obtain an ideal division if a plurality of reflected beams is divided on the intersect of the optical axes. However, there are cases where that intersect is within the lens, or the intersect moves due to tracking movement as a result of a focus servo of the objective lens, and it is not physically possible to construct a means for deriving a bilateral division in a stable manner.